The preparation processes for polycarbonate are known in the literature and described in many applications:
For the preparation of polycarbonates by the interfacial or melt transesterification process, reference is made, for example, to “Schnell”, Chemistry and Physics of Polycarbonates, Polymer Reviews, Vol. 9, Interscience Publishers, New York, London, Sydney 1964, p. 33 ff and to Polymer Reviews, Volume 10, “Condensation Polymers by Interfacial and Solution Methods”, Paul W. Morgan, Interscience Publishers, New York 1965, Chap. VIII, p. 325 and EP-A 971790.
After their preparation, melt polycarbonates may contain catalytically active, basic impurities. These may be caused on the one hand by slight impurities in the starting substances that have not been separated off, by basic residues of thermally decomposable catalysts that have not been separated off, or by stable basic catalyst salts that have not been separated off. Thermally decomposable catalysts are to be understood as being, for example, so-called onium salts. Thermally stable catalysts are to be understood as being, for example, alkali or alkaline earth salts. Such basic substances in the polycarbonate are highly undesirable because they support a catalytic activity of the material.
Accordingly, when additives are mixed into the polycarbonate melt, basic substances may support chemical reactions of the additives, such as, for example, incorporation into the polymer chain and degradation of the additives, which adversely affect the effectiveness of the additives. The thermal reduction of the residual monomers at constant molecular weight, which is conventional in the case of optical melt polycarbonate types, is also not successful in the presence of catalytically active, basic impurities (literature reference LeA 36 697). Catalytically active compounds in the polycarbonate lead further to degradation reactions of the polycarbonate during conventional steps of further processing, such as, for example, injection molding.
For this reason, inhibitors are conventionally added to the polycarbonates. Inhibitors are understood as being all compounds that inhibit the kinetics of chemical reactions, so that quality-reducing changes in the polymer are avoided.
The deactivation of the catalytically active, basic impurities in the melt polycarbonate with the aid of acidic compounds and their esters is known in the literature. It is extremely important in this case that the inhibitors used do not form excess free acids, because these likewise support chemical reactions of the polymers, for example with the additives.
In DE-A 1 031 512, Schnell et al. describe the neutralisation of basic catalysts by addition of acidic components. It is also mentioned therein that, after the neutralisation, it is possible to remove excess acid that has been used by applying a vacuum.
EP-A 435 124 describes the deactivation of alkaline catalysts in the melt polycarbonate by addition of an acid or a simple acid ester of an acid that contains a sulfur atom. In this case, the excess acid is neutralised again by addition of epoxides, before reduction of residual monomers in vacuo. Examples of acidic components that are disclosed include phosphoric acid, toluenesulfonic acid, methyl tosylate and ethyl tosylate.
DE-A 4 438 545 describes mixing a melt polycarbonate with an acidic component, or an ester thereof, having a pKa value <5 in order to neutralise the basic transesterification catalyst before reduction of the residual monomers in vacuo. Phosphoric acid, toluenesulfonic acid and corresponding esters are described as examples of acidic components.
In WO 00 07799, before reduction of the residual monomers of a melt polycarbonate in vacuo, the alkaline catalysts are inhibited by addition of onium salts, such as tetraalkylphosphonium and tetraalkylammonium salts, of dodecylbenzenesulfonic acid.
From WO 02 46272 there is known the use of a combination of S-containing quenchers with phosphoric acid and water and glycerol monostearate (GMS) for melt polycarbonate. In this manner, with precisely matched addition, the residual monomers may be removed, and GMS may be added as mold release agent without an undesired secondary reaction. The effective quenchers that are described include simple alkyl-benzene- and toluene-sulfonic acid esters as well as phosphonium and ammonium salts of p-substituted benzenesulfonic acids. Butyl tosylate is preferably used.
Many of the described deactivators, in particular the free acids and readily cleavable esters, have the disadvantage of corrosive properties at high temperatures and concentration, as may occur, for example, in the case of industrial metering of the inhibitors. It is highly advantageous to use inhibitors that do not affect the materials of the apparatus, in order to prevent particles, metal cations and deficiencies in terms of safety. Furthermore, the majority of the described quenchers are volatile under the conditions conventionally employed in the units for residual monomer removal in the case of melt polycarbonate and in the case of metering into the molten stream of the melt polycarbonate. If the residual monomers are removed with relatively long residence times at elevated temperatures and in vacuo, a large part of the quenchers may be lost and the efficiency of the quenchers may thus be greatly reduced. The indispensable constant and clean metering of the quenchers into the melt stream, with continuous mixing, is likewise made considerably more difficult by high volatility. The added components, such as, for example, phosphoric acid, may additionally be separated from the polycarbonate with the other volatile constituents in the subsequent degassing step and may become concentrated in the installation, leading to damage to the installation as a result of corrosion. When the components that have been separated off, including the components that inhibit the activity of the catalyst, are fed back into the installation circuit, disadvantageous effects on the implementation of the reaction are additionally to be expected. For example, a catalyst quencher may be fed back into the polycarbonate preparation process and thus inhibit the progress of the reaction. Corrosive damage may also occur in the evaporating system, in which the compounds that have been separated off are worked up.
Although quaternary onium salts of para-substituted benzenesulfonic acids, as described in WO 00 07799, are less volatile, they exhibit a striking disadvantage owing to their low solubility in solvents that are preferably used. According to WO 00 07799, they must therefore be dispersed in water in a complex operation for the purposes of metering. The continuously constant metering of a suspension in extremely small amounts is regarded as difficult from an industrial point of view. Solvents that are preferably used are understood as being water and solvents that are inherent in the process, such as phenol.
A further disadvantage of many acid ester quenchers is that they too rapidly generate large amounts of free acids. An excess of free acid catalyses reactions of the polycarbonates with additives, for example, or even promotes reverse reactions of the polycarbonate with phenol, with the liberation of diphenyl carbonate. On the other hand, small amounts of excess ester-bonded acid, which release the free acid very slowly when exposed to heat during further processing of the quenched polycarbonate, are thoroughly desirable. They increase the thermal load capacity of the polycarbonate.
The object was therefore, starting from the prior art, to find inhibitors for quenching catalytically active impurities in the polycarbonate, which inhibitors are not corrosive and have low volatility and at the same time may readily be dissolved and metered in solvents that are inherent in the process. Likewise, the inhibitors should never generate relatively great excesses of free acid in the polycarbonate, in order to avoid degradation reactions of polycarbonate with formation of carbonates or to suppress reactions with the additives. Instead, slow generation of the free acids is desirable. It is particularly desirable that the inhibitor should not form all the possible free acid completely during incorporation into the polycarbonate and any subsequent steps. In this manner, it may exhibit activity again during further processing after granulation of the quenched polycarbonate, such as, for example, injection molding.